Bioenergy from agricultural residues in Ghana

There are strong incentives for increased bioenergy production in Ghana, since it may bring energy self-sufficiency for farmers and communities, cleaner fuels, and the possibility for closing the nutrient-cycle. Therefore, this PhD thesis is investigating production of residue-based ethanol and biomethane under Ghanaian conditions.

Detailed characterisations of thirteen of the most common agricultural residues in Ghana are presented, enabling estimations of theoretical bioenergy potentials and identifying specific residues for future biorefinery applications. When aiming at residue-based ethanol production, it is advised to utilise starchy residue at first (peelings of yam, cassava and plantain), since these hold the largest potentials per unit of mass while they have simpler conversion routes.

Furthermore, only residues concentrated at processing facilities are applicable for ethanol production due to poor rural infrastructure, spatial distribution of the residues and the lack of tradition in collecting residues. Pretreatment methods aimed at low-tech production of cellulosic ethanol from West African biomasses are assessed.

Evaluated on the overall ethanol yield, the low-tech pretreatment methods (boiling-, soaking in aqueous ammonia-, and white rot fungi pretreatments) are viable alternatives to the high-tech method hydrothermal treatment. However, these methods are not as versatile as hydrothermal treatment as they each have satisfactory effect on only a few of the biomasses.

Silage pretreatment is also assessed but is not adequate as stand-alone pretreatment of dry lignified biomass. However, combined with hydrothermal treatment, silage treatment decreases the optimal pretreatment temperature significantly, thereby reducing the energy inputs for hydrothermal pretreatments.   It is recommended to pursue increased implementation of anaerobic digestion in Ghana, as the first bioenergy option, since anaerobic digestion is more flexible than ethanol production with regard to both feedstock and scale of production.

If possible, the available manure and municipal liquid waste should be utilised first as these are verified substrates for low-tech systems. Beside manure, the most recommendable agricultural feedstock for anaerobic digestion are processing residues with high biomethane potentials (BMP) such as starchy peelings, cocoa husks, maize husks and maize cobs.

Biomethane based on agricultural processing residues, manure and municipal liquid waste can theoretically replace approximately 20% of current utilisation of heat energy in households. However, a need is revealed for resilient small-scale anaerobic digestion solutions, designed for utilising agricultural residues under manure- and water shortage.

A novel model for estimating BMP from compositional data of lignocellulosic biomasses is derived. The model is based on a statistical method not previously used in this area of research and the best prediction of BMP is: BMP = 347 xC+H+R – 438 xL + 63 DA , where xC+H+R is the combined content of cellulose, hemicellulose and residuals, xL isthe content of lignin while DA is describing the applied analysis method, DA = (0, Forage analysis method) or (1, Fibre analysis method.) Potential bioenergy solutions should be a part of the fragile equilibria in the Ghanaian agricultural system.

Additionally, current data on available crop residues should be used with caution to predict a future situation since 1) the current utilisation of residues is not clear, 2) changes in infrastructure and farming practices are expected, and 3) a growing bioenergy sector will bring self-inflicting changes to its surroundings.